Relating to Power Adaptors

ABSTRACT

A power adaptor ( 20 ) is provided, comprising an input ( 22 ) for connection to an AC power supply, and a resonant circuit ( 34 ) coupled to the input ( 22 ) that provides an output suitable for driving a load ( 50 ). The capacitance and inductance of the resonant circuit ( 34 ) are selected to provide a pre-determined change in effective voltage, and a corresponding pre-determined change in effective current, between the input ( 22 ) and the output ( 24 ) of the power adaptor ( 20 ).

This invention relates to power adaptors, and in particular poweradaptors suitable for providing a change in effective voltage betweenthe input and the output of the power adaptor.

In domestic applications, the voltage available from the mains supply istypically 120V-240V AC, at frequencies of 50 or 60 Hz. Where a poweradaptor is adapted to drive a low voltage load, such as a 10V load, theratio between the voltage available at the supply and the voltagerequired to drive the load is between 10 and 20. With such a largeratio, conventional switched-mode power adaptors used to drive lowvoltage loads become very inefficient because the switching operates atsmall duty ratios with very short conduction times and current waveformshaving high peak to average ratios.

It is common therefore in prior art power adaptors to include a magnetictransformer with a turns ratio suitable to create a step down in voltageand a corresponding step up in current. In some conventional powersupplies, this magnetic transformer is provided at the input to thepower adaptor, such that the entire power adaptor operates at lowervoltage. In such cases, the magnetic transformer operates at a supplyfrequency (50 or 60 Hz) and is relatively bulky and expensive. In othercases, the magnetic transformer is integrated as part of the switchingcircuit, allowing it to operate at the switching frequency of theelectronic components in the power adaptor. Such circuits therefore havethe advantage that the magnetic transformer can be made smaller.

Nevertheless, all of these prior art arrangements require a magnetictransformer, which is rather bulky and inefficient.

There has now been devised an improved power adaptor which overcomes orsubstantially mitigates the above-mentioned and/or other disadvantagesassociated with the prior art.

According to the invention, there is provided a power adaptor comprisingan input for connection to an AC power supply, and a resonant circuitcoupled to the input that provides an output suitable for driving aload, wherein the capacitance and inductance of the resonant circuit areselected to provide a pre-determined change in effective voltage, and acorresponding pre-determined change in effective current, between theinput and the output of the power adaptor.

The power supply according to the invention is advantageous principallybecause the power adaptor provides a pre-determined change in effectivevoltage, and a corresponding pre-determined change in effective current,between the input and the output of the power adaptor, without any needfor a magnetic transformer. The power adaptor may therefore be adaptedto drive a low voltage load from a higher mains AC supply, without theneed for a magnetic transformer, and without the need for an electronicswitching circuit operating at small duty ratios (short on-timesrelative to switching periods) and having current waveforms with highpeak to average ratios. In particular, the power adaptor may be adaptedto connect to a high voltage AC mains supply (eg 110V or 230V AC, atfrequencies of 50 Hz or 60 Hz), and provide an output suitable fordriving a low voltage solid state light source (eg 10-20V).

The capacitance and inductance of the resonant circuit may be selectedto provide a pre-determined decrease in effective voltage, and acorresponding pre-determined increase in effective current, between theinput and the output of the power adaptor. Alternatively, thecapacitance and inductance of the resonant circuit are selected toprovide a pre-determined increase in effective voltage, and acorresponding pre-determined decrease in effective current, between theinput and the output of the power adaptor.

In presently preferred embodiments, the capacitance and inductance ofthe resonant circuit are selected to provide a pre-determined ratiobetween the effective voltage at the input of the power adaptor and theeffective voltage at the output of the power adaptor, and apre-determined ratio between the effective current at the input of thepower adaptor and the effective current at the output of the poweradaptor.

The resonant circuit is preferably configured to provide an outputhaving an increased or decreased effective current relative to theeffective current through a resonant inductor, such that the output hasa correspondingly decreased or increased effective voltage relative tothe effective voltage of the power supply. The output preferably has aneffective current that is increased or decreased relative to the currentthrough the resonant inductor by a factor of at least two, and mostpreferably by a factor of at least five. In addition, the effectivevoltage decrease or increase of the output relative to the AC powersupply is preferably by a factor of at least two, and most preferably bya factor of at least five.

The pre-determined changes in effective voltage and effective currentbetween the input and the output of the power adaptor are preferablyachieved with no assistance from a magnetic transformer. Indeed, thepower adaptor may be devoid of any magnetic transformers, other thansignal or power supply transformers, which may be present in the poweradaptor.

The pre-determined changes in effective voltage and effective currentbetween the input and the output of the power adaptor may be achievedwith no assistance from a transformer. However, the power adaptor may beprovided with a piezoelectric transformer that isolates the output fromthe input of the power adaptor. In this arrangement, the piezoelectrictransformer may provide a further pre-determined change in effectivevoltage, and a further pre-determined change in effective current,between the input and the output of the power adaptor.

Where the power adaptor is provided with a piezoelectric transformer,the piezoelectric transformer may be arranged to provide at least someof the capacitance of the resonant circuit. In one embodiment, thepiezoelectric transformer provides all of the capacitance of theresonant circuit. The inclusion of a piezoelectric transformer thereforeoffers several advantages when incorporated into a power adaptor havinga resonant circuit. Hence, according to a further aspect of theinvention, there is provided a power adaptor comprising an input forconnection to an AC power supply, a resonant circuit coupled to theinput that provides an output suitable for driving a load, and apiezoelectric transformer that isolates the output from the input of thepower adaptor.

The output is preferably suitable for driving a constant current load,such as a solid state light source. The resonant circuit is preferablytherefore configured to provide an output having a substantiallyconstant voltage, which is pre-determined for a particular effectivevoltage of the AC power supply. In particular, the resonant circuit ispreferably configured to either boost (ie raise) or buck (ie lower) theactual voltage received at the input of the power adaptor, in order toprovide a substantially constant voltage, which is pre-determined for aparticular effective voltage of the AC power supply, for the majority ofthe input AC cycle.

This arrangement of the present invention therefore enables the outputto have a substantially constant voltage, which is pre-determined for aparticular effective voltage of the AC power supply, for a greaterproportion of the input AC cycle than that provided by a magnetictransformer. This arrangement therefore enables the power adaptor tohave a lower bulk storage capacitance than power adaptors that utilisemagnetic transformers to provide a pre-determined change in effectivevoltage.

This arrangement of the present invention is suitable for driving fixedloads. However, the power adaptor may also be adapted to drive variableloads. In particular, the power adaptor may be adapted to turn off theoutput, when the load is insufficient to be driven by the outputprovided by the power adaptor, and then turn on the output when the loadis sufficient to be driven by the output provided by the power adaptor.

In a particularly preferred arrangement for providing an output that issuitable for driving a constant current load, the resonant circuit is anLCL series-parallel resonant circuit.

By “LCL series-parallel resonant circuit” is meant a resonant circuitcomprising a first inductor and a first capacitor in series, and aparallel load leg including a second inductor. The first inductor andfirst capacitor are preferably connected in series between two inputterminals of the resonant circuit, and the resonant circuit preferablycomprises a load leg connected in parallel across the first capacitor,wherein the load leg comprises the second inductor and an output fordriving the load, which are connected in series. In particular, the LCLresonant circuit preferably has input terminals and output terminalswith a first inductor L1, connected from a first input terminal througha common point with second inductor L2, to a first output terminal, thesecond input terminal being directly connected to the second outputterminal, and a capacitor C1, connected between the common point betweenthe two inductors and the direct connections between second terminals ofinput and output. The input terminals are preferably adapted to bedriven from a high frequency inverter. Any of the first inductor, thefirst capacitor and the second inductor may comprise a single inductiveor capacitive component or a combination of such components.

The resonant circuit is preferably adapted such that at one of itsresonant frequencies, the power adaptor provides a constant currentoutput, at a given effective input voltage, and the resonant circuit ispreferably driven at that resonant frequency or a sub-harmonic thereof,or sufficiently near to that resonant frequency or a sub-harmonicthereof for the power adaptor to be suitable for use with a constantcurrent load, such as a solid state light source. In particular, thefirst and second inductors are preferably selected such that thereactance X_(L1) of the first inductor and the reactance X_(L2) of thesecond inductor are substantially equal in magnitude, and aresubstantially equal in magnitude to the reactance X_(C1) of the firstcapacitor. In particular, X_(L1)≈X_(L2)≈−X_(C1) in presently preferredembodiments.

When the chosen components satisfy these conditions, at a given inputvoltage, the current delivered to a load will be constant, independentof the load connected to the power adapter. Furthermore, variation ofthe input voltage would directly control the magnitude of the constantcurrent delivered to the load. When driving a constant voltage load,such as LEDs, the power delivered to the load would therefore bedirectly proportional to the input voltage, without requiring anyfeedforward or feedback control.

Where the LCL series-parallel resonant circuit is adapted to provide aconstant current output, the capacitance of the LCL series-parallelresonant circuit is preferably selected with a reactance X_(C1) to matcha required load resistance R_(L) and a required, relatively higher,input resistance R_(in) for the resonant circuit. The first capacitor ispreferably selected using the following equation:

X _(C1)=√{square root over (R _(in) R _(L))}  (1)

where X_(C1) is the reactance of the first capacitor. The reactance ofthe first capacitor is preferably therefore equal to the square-root ofthe product of the required load resistance R_(L) and the required inputresistance R_(in) for the resonant circuit.

The output of the power adaptor is preferably therefore adapted to beconnected to a load of apparent impedance R_(L) the value of the firstand second inductors, and the first capacitor, L1, L2 and C1, beingchosen such that at least one frequency, the reactances of L1, L2 and C1are approximately similar in magnitude and that at least one frequencythe apparent impedance seen at the input terminals R_(in) is transformedby the LCL resonant circuit to be approximately equal to the square ofthe reactance of the capacitor X_(C1) divided by the apparent impedanceof the load, R_(L).

This selection of the inductance and capacitance of the resonant circuittherefore provides a pre-determined change in effective voltage betweenthe input and the output of the power adaptor.

In presently preferred embodiments, the power adaptor according to theinvention comprises an input for connection to a mains AC power supply,and the resonant circuit provides an output suitable for driving a solidstate light source. As discussed above, the resonant circuit ispreferably an LCL series-parallel resonant circuit.

The use of an LCL series-parallel resonant circuit is particularlyadvantageous when the power adaptor is adapted to provide an outputsuitable for driving a solid state light source. In particular, the LCLseries-parallel resonant circuit may be adapted to provide a constantcurrent output suitable for driving a solid state light source, which isnot dependent upon the load, and does not require any form of feedbackor complex control. A power adaptor including an LCL series-parallelresonant circuit may therefore provide a much more efficient transfer ofpower from the mains power supply to the solid state light source, incomparison to prior art power adaptors, and the power adaptor may bemore compact and have a lower manufacturing cost than prior artadaptors. In addition, the power at the output of the power adaptorwould typically reduce as the input power reduces, and hence a poweradaptor including an LCL series-parallel resonant circuit is suitablefor use with conventional power reducing devices associated with themains power supply. Hence, according to a further aspect of theinvention, there is provided a power adaptor for a solid state lightsource, the power adaptor comprising an input for connection to a mainspower supply, and an LCL series-parallel resonant circuit coupled to theinput that provides an output suitable for driving the solid state lightsource.

The resonant circuit is preferably adapted to provide, at a given inputvoltage, a constant current output. The power delivered to the outputpreferably therefore varies with variation of the voltage at the input,with no need for any control. In particular, the magnitude of theconstant current is preferably proportional to the input voltage.Furthermore, the resonant circuit is preferably adapted to provide, at agiven input voltage, a constant current output that is independent ofthe load. In order to achieve these characteristics, the resonantcircuit is preferably adapted such that one of its resonant frequenciesprovides these properties, and the resonant circuit is preferably drivenat that resonant frequency, or sufficiently near to that resonantfrequency for the power adaptor to be suitable for use with a solidstate light source.

Nevertheless, it has been found that by driving the resonant circuit ata sub-harmonic of the resonant frequency, the power factor and/orefficiency of the power adaptor may be improved. Most preferably, theresonant circuit is driven at a sub-harmonic of 1/x, where x is an oddnumber, for example, 1/3, 1/5 or 1/7. Driving the resonant circuit at asub-harmonic of the resonant frequency has the advantage that theswitching frequency and switching losses of the resonance drive circuitmay be reduced, thereby improving the efficiency of the power adaptor.In most prior art resonant circuits, driving the circuit at asub-harmonic would reduce the power. However, the LCL series-parallelresonant circuit may be adapted to have one of its resonant frequenciesat 0 Hz, as discussed in more detail below, which allows low frequencycurrents to pass through to the load. Hence, the current passing throughthe resonant circuit and the power delivered to the load does not changesubstantially if the circuit is driven at a sub-harmonic of the resonantfrequency.

The LCL series-parallel circuit maybe adapted to have three resonantfrequencies, a first resonant frequency at 0 Hz, ie DC current, a secondresonant frequency that provides, at a given input voltage, a constantcurrent output that is independent of the load, and a third resonantfrequency that provides, at a given input voltage, a current that varieswith load. These resonant frequencies are preferably achieved byselecting the first inductor, the second inductor and the firstcapacitor, such that the reactances of those components aresubstantially equal. The third resonant frequency may be adapted toprovide a significantly greater power at the output, relative to thesecond resonant frequency. A controller of the power adaptor maytherefore be adapted to switch between the different resonantfrequencies in order to utilise their different characteristics. Forexample, a controller of the power adaptor may be adapted to switchbetween the second and third resonant frequencies to compensate for achange of input voltage, eg between 230V and 110V AC. Further examplesof such control include loading a TRIAC in the lighting system atcritical points, and altering the power factor and/or regulation of thepower adaptor.

As the voltage at the input varies sinusoidally, the current drawn fromthe input by an LCL series-parallel resonant circuit, configured asdescribed above, will inherently follow a square shape. However, thewaveform of the current drawn from the input by the resonant circuit maybe modified by a controller of the power adaptor. The power adaptor maytherefore include a controller adapted to determine the waveform of thecurrent drawn from the input by the resonant circuit. In particular, thecontroller may be adapted to modify the waveform of the current thatwould inherently be drawn by the resonant circuit, such that thewaveform of the current drawn from the input is more similar in shape tothe waveform of the voltage at the input. In particular, the currentdrawn by the resonant circuit may have a waveform that is generallysinusoidal, but with flattened peaks.

The resonant circuit is preferably driven by a resonance drive circuit,which provides a resonance drive signal to the resonant circuit. Theresonance drive signal is preferably an alternating signal, and ispreferably provided by an oscillator that may control two or fourelectronic switches, eg FETs. The resonance drive signal typically hasthe form of a square wave. The purpose of the drive circuit is to excitethe resonant circuit with an alternating voltage, the alternatingvoltage typically consisting of blocks of positive and negative voltage.The electronic switches are typically connected together in the form ofa full bridge inverter (4 switches) or a half bridge inverter (2switches).

As discussed above, the power adaptor may be adapted to modify thewaveform of the current that would inherently be drawn by the resonantcircuit, and in particular modify the shape and/or size of thatwaveform. In particular, a resonance drive signal may be provided to theresonant circuit, wherein the resonance drive signal is adapted todetermine the desired input current waveform. For instance, theresonance drive signal may be adapted in a variety of ways including,but not limited to, any of the following including combinations thereof:(i) introducing a dead-band between half-cycles or full cycles of thealternating drive signal, (ii) varying the frequency of the drivesignal, and (iii) missing cycles of the alternating drive signal.

Where the resonance drive signal is adapted by missing cycles of thealternating drive signal, these missing cycles may be arranged in adiscontinuous arrangement, in a single continuous group, or in aplurality of continuous groups, for each mains supply cycle. Where themissing cycles are arranged in a plurality of continuous groups, thenumber of continuous groups for each mains supply cycle is preferablyselected to be appropriate for the output power, and hence may bevariable with the output power.

As discussed below, the power adaptor may be adapted to control thelight output from the solid state light source. In this embodiment, theresonance drive signal is preferably variable, for example by acontroller, in order to determine the light output from the solid statelight source. The resonance drive signal is preferably also adapted tooptimise the power factor and/or efficiency of the power adaptor.

Alternatively, where the power adaptor is configured such that the lightoutput from the solid state light source is only controllable by varyingthe power available at the input of the power adaptor, the resonancedrive signal may be predetermined, preferably to optimise the powerfactor and/or efficiency of the power adaptor.

Any controller of the power adaptor, as discussed above, is preferablyadapted to control the resonant drive signal provided to the resonantcircuit, in order to determine the waveform of the current drawn fromthe input by the resonant circuit. This controller of the power adaptormay be provided by an integrated circuit, such as a microprocessor, ananalogue electronic circuit, or any combination of analogue and digitalelectronics. Indeed, the controller of the power adaptor may be anapplication specific, integrated circuit, which may be manufactured atvery low cost. In this configuration, the oscillator of the drivecircuit may also form part of the integrated circuit, or may be aseparate circuit.

The determination of the frequency at which the resonant circuit isdriven may be used to calibrate the power adaptor for improvedefficiency. Alternatively, the frequency at which the resonant circuitis driven may be varied during use, in order to vary the power beingsupplied to the solid state light source.

The output for driving the solid state light source may be isolated fromthe resonant circuit, particularly for applications in which users wouldhave access to the solid state light source and/or associated circuitry.In this case, the power adaptor preferably comprises a piezoelectrictransformer to provide this isolation.

The resonant circuit may also include a pair of potential dividingcapacitors, to which the first capacitor is connected. Alternatively,where the resonance drive circuit contains four electronic switches (egFETs) arranged to create two switching legs (eg a “H-bridge”), as asingle phase inverter, the pair of capacitors could be replaced by asingle capacitor. These capacitors are preferably Y capacitors.

In another embodiment, the resonance drive circuit comprises twoelectronic switches (eg FETs) connected between the LCL series-parallelresonant circuit and ground, ie two “low-side” switches. These twolow-side switches preferably each alternate between ON and OFF, which afirst switch being ON whilst a second switch is OFF, and vice versa.This arrangement is particularly advantageous where the switches aredriven by a low voltage controller, such as an integrated circuit.

In this embodiment, the first resonant inductor of the LCLseries-parallel resonant circuit preferably comprises two inductors, oneconnected to one end of the first capacitor, and the other connected tothe other end of the first capacitor. In this arrangement, one of thesetwo inductors will be active in the positive half cycle of the supply,and the other of these two inductors will be active in the negative halfcycle of the supply. In one embodiment, these two inductors are woundabout a common core, such that the first resonant inductor of the LCLseries-parallel resonant circuit is a three terminal inductor.

The power adaptor may draw current from the input as a function of thevoltage at the input in order that the power adaptor appears as aresistive load to the mains supply. This is preferably achieved by: (i)minimising the capacitance at the input of the power adaptor, (ii)drawing a current waveform from the input that is substantially in phasewith the voltage waveform at the input, and/or (iii) drawing currentthat is substantially proportional to the voltage. These features reducecurrent distortion and harmonic currents drawn from the mains supply,and increase the efficiency and power factor of the power adaptor byremoving the capacitive load presented to the mains supply. Indeed,these features enable the power adaptor and connected solid state lightsource to be presented to the mains supply as a conventional filamentlight source.

Alternatively, the power adaptor may draw power from the input as afunction of the voltage at the input, such that the power adaptor doesnot appear as a resistive load to the mains supply.

The solid state light source is preferably a Light Emitting Diode (LED),or a series of two or more LEDs. The power adaptor preferably includesone or more diodes at its output, eg a diode bridge, to ensure that noreverse currents are present that could damage the solid state lightsource.

Any control circuitry of the power adaptor may be powered by anintegrated power supply. Alternatively, the control circuitry of thepower adaptor may be powered by a connection to one of the inductors ofthe resonant circuit, for instance a connection to a winding coupled tothat inductor.

Where the power adaptor includes an integrated power supply, theintegrated power supply preferably draws power directly from the mainspower supply, most preferably via the input of the power adaptor. Inparticular, the integrated power supply is preferably a constant currentpower supply, such as a switch mode constant current regulator, whichpreferably does not cause excessive inrush and is low in cost. Thecontrol circuitry is preferably adapted to shut itself down during theoff periods of a mains cycle, for example when the power adaptor isconnected to a TRIAC or similar device, so that the constant currentdevice can be low in power and hence the efficiency high.

The power adaptor preferably also includes a fault detection circuitthat disables the resonant circuit, preferably by removing theoscillating drive signal, in the event that the load is removed, whichmay be caused by failure or disconnection of the light source, forexample. The fault detection circuit preferably connects an output ofthe resonant circuit with the controller. This fault detection circuitis a feedback circuit, but it preferably draws minimal power from theoutput of the resonant circuit during normal operation, and hence shouldnot be confused with an active feedback circuit that regulates the poweroutput. The fault detection circuit would be active during a faultcondition only, and is not essential for controlling the output powerduring normal use.

The power adaptor may include a filter at its input for reducingharmonic currents drawn from the mains supply. The filter may comprise asmall non-electrolytic capacitor-inductor network. The power adaptorpreferably also includes a rectifier at its input that converts theinput waveform to one of constant polarity. Most preferably, therectifier is a full wave rectifier that reverses the negative (orpositive) portions of the alternating current waveform. Nevertheless,there is no need for the power adaptor to provide a steady DC signal atthe input of the LCL series-parallel resonant circuit, and hence a bulkstorage capacitor (also known as a reservoir capacitor or smoothingcapacitor) is preferably not provided between the input of the poweradaptor and the LCL series-parallel resonant circuit. Hence, the poweradaptor is preferably substantially free of bulk storage capacitancebetween the input of the power adaptor and the resonant circuit. Indeed,the power adaptor is preferably substantially free of electrolyticcapacitors. This enables the supply to be designed with minimalreactance, minimal inrush current, and long life with reduced size andcost relative to prior art power adaptors for solid state lightingsystems. A bulk storage capacitor may be provided at the output of thepower adaptor, but this is not essential for the functioning of thepower adaptor with a conventional solid state light source.

The power adaptor according to the invention is preferably suitable foruse in a lighting system that utilises any power reducing device fordetermining the power available at the input of the power adaptor. Inparticular, the power reducing device may be a variable resistor, suchas a Variac, or a rheostat. The power adaptor may also be adapted tofunction in lighting systems that include a dimmer control utilising SCRphase control or a triac in order to reduce the power available at theinput of the power adaptor. In this case, however, the power adaptor maybe adapted to draw a minimum current from the mains supply to keep theSCR stable during the full mains cycle, unless the lighting unit isswitched off, to ensure the continued functioning of the dimmer control.

A further advantage of the power adaptor according to the invention isthat no monitoring of the voltage at the input, for example by acontroller of the power adaptor, is necessary. Hence, the power adaptoraccording the invention may be devoid of any means for monitoring thevoltage at the input, and in particular the power adaptor may be adaptedsuch that the controller does not receive a signal from the input.

The power adapter may include a controller able to deliver a controlsignal to the resonant circuit for reducing power drawn from the input.However, in other embodiments, the power adaptor does not include acontroller having such a feature. In particular, the power adaptor maybe adapted so that the light output from the solid state light source isonly controllable by varying the power available at the input of thepower adaptor. In particular, the power available at the input of thepower adaptor may be varied using an external device, such as anexternal power reducing device, associated with the mains supply. Thisembodiment is particularly suitable for use with a lighting unitincluding an integral power adaptor, which would be suitable forincorporation into a conventional lighting circuit. In order to maximisethe efficiency of the power adaptor, the power adaptor is preferablyadapted to transfer all power available at the input, save forunavoidable losses, to the output of the power adaptor.

According to a further aspect of the invention, there is provided alighting system comprising a power adaptor as described above and alighting unit including at least one solid state light source.

The lighting unit will typically be provided with a plurality of solidstate light sources. In order to achieve different colours of lightoutput, the lighting unit may include solid state light sources thatemit light of different colours, for example LEDs that emit light ofred, green and blue colour. Furthermore, the lighting unit may alsoinclude LEDs of amber, cyan and white colour in order to raise thecolour rendering index.

The power adaptor and the lighting unit may have a common housing, ormay be housed separately. Indeed, the power adaptor may be adapted toprovide power to a plurality of lighting units, each lighting unitincluding a plurality of solid state light sources. Furthermore, thelighting system may include a plurality of such power adaptors. Thelighting system may also include a power reducing device, such as avariable resistor, a rheostat or a dimmer control that utilises SCRphase control.

The power adaptor according to the invention is particularly suitablefor use with a lighting unit including an integral power adaptor, whichwould be suitable for incorporation into a conventional lightingcircuit. Hence, according to a further aspect of the invention, there isprovided a lighting unit suitable for direct connection to a mainssupply, the lighting unit comprising a power adaptor as described aboveand one or more solid state light sources, in which the light outputfrom the one or more solid state light sources is controllable byvarying the power available at the input of the power adaptor. In orderto maximise the efficiency of the power adaptor, the power adaptor ispreferably adapted to transfer all power available at the input, savefor unavoidable losses, to the output of the power adaptor.

The lighting unit preferably comprises a housing for accommodating thepower adaptor and the one or more solid state light sources, and aconnector for connecting the input of the power adaptor to the mainssupply. The connector is preferably adapted to connect to a fitting fora conventional filament light bulb. In particular, the lighting unit mayinclude a bayonet or threaded connector. In one embodiment, the lightoutput from the one or more solid state light sources is onlycontrollable by varying the power available at the input of the poweradaptor.

According to a further aspect of the invention, there is provided anelectronic impedance matching circuit incorporating an LCL resonantcircuit with input terminals and output terminals with a first inductorL1, connected from a first input terminal through a common point withsecond inductor L2, to a first output terminal, the second inputterminal being directly connected to the second output terminal, and acapacitor C1, connected between the common point between the twoinductors and the direct connections between second terminals of inputand output, the input terminals being driven from a high frequencyinverter, the output terminals being connected to a load of apparentimpedance RL, the value of the components L1, L2 and C1 being chosensuch that at least one frequency, the reactances of L1, L2 and C1 areapproximately similar in magnitude and that at least one frequency theapparent impedance seen at the input terminals is transformed by the LCLresonant circuit to be approximately equal to the square of thereactance of the capacitor divided by the apparent impedance of theload, RL.

The frequency of operation of the high frequency inverter is preferablyclose to the frequency where the reactances of L1, L2 and C1 areapproximately similar.

The electronic impedance matching circuit is preferably adapted to driveone or more LEDs with a voltage requirement which is substantially lowerthan the input supply voltage.

Alternatively, however, the electronic impedance matching circuit may beadapted to charge a battery with a voltage requirement which issubstantially lower than the input supply voltage, or may be adapted todrive an electric motor at more than one speed by dynamically matchingthe varying apparent load impedance to the supply voltage.

A preferred embodiment of the invention will now be described in greaterdetail, by way of illustration only, with reference to the accompanyingdrawings, in which

FIG. 1 is a schematic diagram of a power adaptor according to theinvention;

FIG. 2 is a schematic diagram of a resonant circuit, including aresonance controller and a resonance drive circuit, that forms part ofthe power adaptor of FIG. 1;

FIG. 3 is a schematic diagram of the resonant circuit of FIG. 2,including an alternative resonant drive circuit;

FIG. 4 is a schematic diagram of a second alternative to the circuitshown in FIG. 2;

FIG. 5 is a schematic diagram of a third alternative to the circuitshown in FIG. 2;

FIG. 6 is a schematic diagram of a fourth alternative to the circuitshown in FIG. 2;

FIG. 7 is a schematic diagram of a fifth alternative to the circuitshown in FIG. 2; and

FIG. 8 is a schematic diagram of a lighting system according to theinvention.

FIG. 1 shows a power adaptor 20 according to the invention. The poweradaptor 20 comprises an input 22 for drawing electrical power from themains circuit, and an output 24 for providing electrical power to thethree LEDs 60 a,60 b,60 c of the solid state lighting unit 50. The poweradaptor 20 includes a filtering and rectifying circuit 30 at the input22, such that the AC voltage waveform drawn from the mains circuit issupplied to the remainder of the power adaptor circuitry as a full-waverectified waveform (DC+).

The power adaptor 20 also includes a low power, auxiliary power supply32, and a resonant circuit 34 including a resonance controller 40 and aresonance drive circuit 42, which are described in more detail belowwith reference to FIG. 2. The low power, auxiliary power supply 32provides a low power DC output (+V) for powering the integrated circuitsof the resonance controller 40 and the resonance drive circuit 42. Thisprovides a stable power supply to the integrated circuits of the poweradaptor to ensure stable functioning of those circuits. It is noted thatin other embodiments, the integrated circuits of the power adaptor arepowered by connections to additional windings coupled to one of theinductors of the resonant circuit, and hence the auxiliary power supply32 is omitted.

The resonant circuit 34, including the resonance controller 40 and theresonance drive circuit 42, is shown in FIG. 2. The resonance controller40 includes a control circuit and is adapted to control the resonancedrive circuit 42. In particular, the resonance controller 40 has anoutput for supplying a control signal to the resonance drive circuit 42,which determines the form of the current drawn from the input by theresonant circuit 34. It is noted that in other embodiments, theresonance drive circuit 42 is self-oscillating, and the control circuitis omitted altogether.

The resonant circuit 34 has the form of an LCL series-parallel resonantcircuit (L1, C1 and L2). The resonance drive circuit 42 is adapted todrive the LCL series-parallel resonant circuit with a square wavedriving signal. This square wave signal is generated by two electronicswitches, eg FETs, connected to a first end of the resonant circuit, andassociated drive circuitry 44. The FETs are controlled by the resonancecontroller 40. The output of the resonant circuit 34 is rectified usinga diode bridge, and then smoothed by a capacitor (C5) at the output ofthe rectifier, so as to form an output suitable for driving the LEDs 60a,60 b,60 c. The capacitors C2 and C3 create a connection point for thesecond end of the resonant circuit, substantially midway in voltagebetween DC+ and 0V.

Alternatively, the resonance drive circuit 42 contains four electronicswitches (eg FETs) arranged to create two switching legs (in a“H-bridge”), as a single phase inverter, as illustrated in FIG. 3. Inthis embodiment, the capacitors C2 and C3 have been be replaced by asingle capacitor (C2) connected between DC+ and 0V. The circuit cannotoperate with no capacitance across the DC supply, as a small amount ofcapacitance is required to protect the switches from overvoltage damageduring switching transients.

The LCL series-parallel resonant circuit is configured such that at achosen frequency, the reactance of L1 (X_(L1)), the reactance of C1(X_(C1)) and the reactance of L2 (X_(L2)) are substantially equal. Inthis configuration, the LCL series-parallel resonant circuit has twonon-zero resonant frequencies. The frequency at which the reactances areequivalent will be one of the two non-zero resonant frequencies. Whendriving the resonant circuit at this frequency, the resonant circuitsupplies a constant current to the output, and hence to the LEDs 60 a,60b,60 c, regardless of the load. The magnitude of the constant current isproportional to the input voltage. This resonant frequency is

$\begin{matrix}{\omega_{1} = {+ \frac{1}{\sqrt{L_{S}C_{P}}}}} & (2)\end{matrix}$

The resonance controller 40 and the resonance drive circuit 42 istherefore adapted to excite the LCL series-parallel resonant circuitclose to this resonant frequency, ω₁. As a consequence of driving theresonant circuit close to the resonant frequency, the switching lossesin the electronic switches are reduced, and hence the efficiency of thecircuit is improved. Further advantages include the reduction ofconducted and radiated electromagnetic interference, and hence thereduction of the expense of necessary filtering and screeningcomponents.

The normal characteristic of this configuration of the LCLseries-parallel resonant circuit is to draw a power which is directlyrelated to input voltage. Without any control, as the voltage at theinput 22 varies sinusoidally, the AC current drawn from the input 22would follow a square shape. However, it is possible to use the on-timemodulation and/or the frequency of the switches to reduce the powerdrawn from the input 22 in the proximity of each zero crossing, andtherefore to improve the input current harmonics. In addition, theoptional capacitor (C5) on the output of the rectifier smoothes thepower delivered to the LED such that the light output will contain lessfluctuation.

A fault detection circuit is preferably provided that includes aconnection between the output of the LCL series-parallel resonantcircuit and a disable pin on the PIC of the resonance controller 40,through resistor R1, and a connection with 0V through resistor R2. Thefault detection circuit draws minimal power. However, in the event thatan LED 60 a,60 b,60 c stops conducting, the associated fault detectioncircuit quickly detects a rise in voltage at the output of the resonantcircuit and causes the resonance controller 40 to shut-off its output tothe resonant drive circuit 42, and hence cause the drive signal to beremoved from the resonant circuit 34. In FIG. 2, the fault detectioncircuit is shown connected between L2 and the diode bridge. However,please note that this circuit could also be connected between thepositive end of the diode bridge and the positive terminal of the output24.

The amount of power delivered to the LEDs 60 a,60 b,60 c can be variedwith the variation of the input mains supply voltage, which makes itsuitable for use with a power reducing device 10.

FIG. 4 shows a further alternative to the circuit shown in FIG. 2, inwhich the resonance controller 40 has been omitted. In this embodiment,the resonance drive circuit 42 consists simply of two electronicswitches, eg FETs, connected to a first end of the resonant circuit, andassociated drive circuitry 44 that is any form of analogue or digitalcircuit capable of providing a suitable drive signal to the electronicswitches. Furthermore, this embodiment does not include any faultdetection circuit (R1 and R2 in FIGS. 2 and 3), or any capacitor (C5FIGS. 2 and 3) at the output of the rectifier.

The power adaptors described above in relation to FIGS. 1-4 are eachadapted to connect to a high voltage power supply (eg 110V or 230V AC,at frequencies of 50 Hz or 60 Hz), and provide an output suitable fordriving a low voltage load, such as a solid state light source (eg10-20V). In particular, the LCL series-parallel resonant circuit of eachpower adaptor is adapted to provide an output having a significantlydecreased voltage, and a significantly increased current, relative tothe power supply, without any need for a magnetic transformer.

The LCL series-parallel resonant circuits of the power adaptor describedin relation to FIGS. 1-4 each have a first terminal and second terminalconnected to a full bridge inverter with four switching devices or ahalf bridge inverter with two switching devices and voltage dividingcapacitors. A first inductor L1, and first capacitor C1 are connected inseries from the first terminal to the second terminal. The load leg ofthe circuit is connected in parallel with the first capacitor C1, theload leg comprising a second inductor L2 in series with a rectifyingmeans to supply unidirectional current to the load while current in theresonant circuit alternates at high frequency.

In such a circuit, the voltage across the first capacitor C1 determinesthe current which is driven through the load leg. It would be expectedtherefore that if the reactance of the first inductor L1 was increased,a greater voltage would be dropped across that component and the voltageacross the load leg would be more closely matched to the lower voltagerequired.

It has been discovered that this is not the case, but it is possible tochoose values for the resonant components L1, L2 and C1 such that thecurrent in the load leg is significantly higher than the current in thefirst inductor L1.

The current in the load leg of this circuit at any frequency is givenby:

$\begin{matrix}{i_{L\; 2} = \frac{X_{C\; 1}V}{{R_{L}X_{L\; 1}} + {R_{L}X_{C\; 1}} + {j\left( {{X_{L\; 1}X_{C\; 1}} + {X_{L\; 2}X_{L\; 1}} + {X_{L\; 2}X_{C\; 1}}} \right)}}} & (3)\end{matrix}$

Where X_(L1), X_(L2), X_(C1) are the reactances of the resonantcomponents L1, L2 and C1, respectively, V is the excitation voltage,R_(L) is the effective resistance of the load and j is the reactivecomponent.

When X_(L1)=X_(L2)=−X_(C1) the above equation simplifies to

$\begin{matrix}{i_{L\; 2} = \frac{V}{- {j\left( X_{C\; 1} \right)}}} & (4)\end{matrix}$

In this configuration, the current in the load is independent of theload, and is proportional to the input supply voltage.

The further step of decreasing X_(C1) results in an increase in the loadcurrent for the same voltage. However, a surprising aspect of thisinvention is that at resonance, the input resistance of the circuit is

$\begin{matrix}{R_{in} = \frac{X_{C\; 1}^{2}}{R_{L}}} & (5)\end{matrix}$

Rearranging,

X _(C1)=√{square root over (R _(in) R _(L))}  (6)

Hence it is possible to choose a value of X_(C1) to match a given loadR_(L) to a required (higher) value of R_(in) such that the current drawnat the input is small and the current delivered to the load is high.

Thus, this embodiment of the invention can drive a low voltage LEDstring from a higher voltage supply by correct choice of capacitors andinductors. The circuit also benefits from the constant current aspectsof this circuit.

As an example an LED string of forward voltage 12V with a currentrequirement of 1 A is to be driven from a 230V AC power supply. Theapparent resistance of the load R_(L) is 12Ω. The power of the load is12 W so the power of the input (assuming no losses) is 12 W. If the halfbridge inverter with split capacitors is used to drive the resonantcircuit, the effective voltage applied on the resonant circuit is ±115V.The required input resistance is therefore approximately 1100Ω. Thevalue of X_(C1) is therefore 115Ω, which corresponds to a capacitance of20 nF at a frequency of 70 kHz. The corresponding values of L₁ and L₂would be 260 μH.

A further embodiment of the power adaptor according to invention isshown in FIG. 5. This embodiment is similar to the previous embodiments,in that it comprises a half-bridge inverter (M1, M2), an LCLseries-parallel resonant circuit (L1, C1, L2), a pair ofpotential-dividing capacitors (C2, C3), and a schottky diode bridge(D1-D4) and a capacitor (C4) at its output. The output is connected toone or more LEDS (two LEDs, LED 1 and LED 2, are shown in FIG. 5), whichare connected in series.

However, this embodiment differs from the previous embodiments in thatthe capacitor (C1) of the LCL series-parallel resonant circuit isdefined by a piezoelectric transformer. The piezoelectric transformercomprises four piezoelectric transformer elements, which are formed of aceramic material, such as PZT (lead zirconate titanate).

The LCL series-parallel resonant circuit including the piezoelectrictransformer is adapted to provide an output having a significantlydecreased voltage, and a significantly increased current, relative tothe power supply. In particular, the power adaptor is adapted to connectto a high voltage power supply (eg 110V or 230V AC, at frequencies of 50Hz or 60 Hz), and provide an output suitable for driving a low voltagesolid state light source (eg 10-20V).

The piezoelectric transformer also isolates the output of the poweradaptor from the input of the power adaptor.

A further embodiment of the power adaptor according to invention isshown in FIG. 6. This embodiment is similar to the previous embodiments,in that it comprises an LCL series-parallel resonant circuit (L1, C1,L2), a pair of potential-dividing capacitors (C2, C3), and a schottkydiode bridge (D1-D4) and a capacitor (C4) at its output. The output isconnected to two LEDs (LED 1 and LED 2), which are connected in series.

However, this embodiment differs from the previous embodiments in thatthe resonance drive circuit comprises two FETs (M1, M2) connectedbetween the LCL series-parallel resonant circuit and ground, ie two“low-side” switches. These two low-side switches each alternate betweenON and OFF, which a first switch being ON whilst a second switch is OFF,and vice versa, so as to create a square-wave driving signal.

Furthermore, the first resonant inductor (L1) of the LCL series-parallelresonant circuit of the previous embodiments has been replaced by twoinductors (L1 a and L1 b), one connected to one end of the capacitor C1,and the other connected to the other end of the capacitor C1. In thisarrangement, one of these two inductors (L1 a) will be active in thepositive half cycle of the resonant frequency at the output, and theother of these two inductors (L1 b) will be active in the negative halfcycle of the resonant frequency at the output.

This embodiment is particularly advantageous in arrangements in whichthe switches (M1, M2) are driven by a low voltage controller, such as anintegrated circuit.

A further embodiment of the power adaptor according to invention isshown in FIG. 7. This embodiment is identical to the embodiment shown inFIG. 6, save for the inclusion of a piezoelectric transformer in anarrangement that corresponds to the arrangement shown in FIG. 5. Thisembodiment combines the advantages discussed above in relation to FIGS.5 and 6.

Finally, FIG. 8 shows a lighting system according to the invention. Thelighting system is connected to a mains circuit including a mains supplyL,N and a power reducing device 10, such as a TRIAC, and comprises apower adaptor 20 according to the invention and a solid state lightingunit 50. The solid state lighting unit 50 comprises three LEDs 60 a,60b,60 c connected in series. The power adaptor 20 is supplied withelectrical power from the mains circuit, and is adapted to provideelectrical power to the LEDs 60 a,60 b,60 c of the solid state lightingunit 50.

1-34. (canceled)
 35. A power adaptor for one or more solid state lightsources comprising an input for connection to an AC power supply, and anLCL series-parallel resonant circuit coupled to the input that providesan output suitable for driving the one or more solid state lightsources, wherein the lighting unit includes a controller and electronicswitches for driving the resonant circuit, the controller being adaptedto use on-time modulation of the switches to determine the waveform ofthe current drawn from the input by the resonant circuit.
 36. A poweradaptor as claimed in claim 35, wherein the LCL series-parallel resonantcircuit has a first resonant frequency that provides, at a given inputvoltage, a constant current output that is independent of the load, anda second resonant frequency that provides, at a given input voltage, acurrent that varies with load, the second resonant frequency providing asignificantly greater power at the output, relative to the firstresonant frequency, and the controller of the power adaptor beingadapted to switch between the resonant frequencies.
 37. A power adaptoras claimed in claim 35, wherein the resonant circuit is configured toprovide an output having a substantially constant voltage, which ispre-determined for a particular effective voltage of the AC powersupply.
 38. A power adaptor as claimed in claim 37, wherein the resonantcircuit is preferably configured to either boost (ie raise) or buck (ielower) the actual voltage received at the input of the power adaptor, inorder to provide a substantially constant voltage, which ispre-determined for a particular effective voltage of the AC powersupply, for the majority of the input AC cycle.
 39. A power adaptor asclaimed in claim 35, wherein the power adaptor is adapted to turn offthe output, when the load is insufficient to be driven by the outputprovided by the power adaptor, and then turn on the output when the loadis sufficient to be driven by the output provided by the power adaptor.40. A power adaptor as claimed in claim 35, wherein the resonant circuitis adapted such that at one of its resonant frequencies, the poweradaptor provides a constant current output, at a given effective inputvoltage, and the resonant circuit is driven at that resonant frequencyor a sub-harmonic thereof, or sufficiently near to that resonantfrequency or a sub-harmonic thereof.
 41. A power adaptor as claimed inclaim 35, wherein the capacitance of the LCL series-parallel resonantcircuit is selected with a reactance X_(C1) to match a required loadresistance R_(L) and a required input resistance R_(in) for the resonantcircuit.
 42. A power adaptor as claimed in claim 41, wherein thecapacitance is selected with a reactance that is equal to thesquare-root of the product of the required load resistance R_(L) and therequired input resistance R_(in) for the resonant circuit.
 43. A poweradaptor as claimed in claim 35, wherein the resonant circuit isconfigured to provide an output having an increased current relative tothe current through a resonant inductor, such that the output has acorrespondingly decreased effective voltage relative to the effectivevoltage of the power supply.
 44. A power adaptor as claimed in claim 35,wherein the controller comprises an integrated circuit.
 45. A lightingunit comprising a power adaptor as claimed in claim 35 and one or moresolid state light sources.
 46. A lighting unit as claimed in claim 45,wherein the lighting unit is suitable for direct connection to a mainssupply, and the light output from the one or more solid state lightsources is controllable by varying the power available at the input ofthe power adaptor.